1. Field of the Invention
This invention relates to an apparatus and method for cutting a window through a tubular casing so as to drill a deviated borehole from an existing casing through geologic formations.
2. Description of the Related Art
It is known, for example, from U.S. Pat. No. 6,648,068 to have a well bore casing from which it is desired to “side track”, and to lower a whipstock and tapered mill combination into the casing, anchor the whipstock to the casing when the whipstock has been appropriately oriented, break a link connecting the mill to the whipstock and to rotate the mill whilst moving it downwardly against the whipstock to cut a window through the casing wall and, thence, to continue cutting through formation in the desired direction.
As disclosed in U.S. Pat. No. 6,648,068, a whipstock may have ramps for moving the mill radially outwardly against an inside wall of the casing and there may be two ramps of about 15° interspaced by a further ramp having an angle of about 3° to a longitudinal axis of the casing. The mill is formed of plural circumferentially disposed radially extending blades, each having a taper of about 15° and the mill blades are faced with cutting material. Located upstream from the tapered mill may be sequentially positioned in the drill string a teardrop mill and a watermelon mill.
It will be understood that in the operation of cutting a window in the casing and sidetracking through formation to a new exploration site that energy production is ceased, thereby leading to a loss of revenue. Thus, it is desired to perform the milling and sidetracking operations as quickly as possible.